Excitation Transfer Cross Sections Research Articles

State-selective electron transfer in He++He collisions at intermediate energies

Electron transfer processes in ${\mathrm{He}}^{+}+\mathrm{He}$ collisions are studied theoretically using a three-electron semiclassical atomic-orbital close-coupling method in a wide energy domain, from 1 to 225 keV/u. Total, state-selective, and angular-differential cross sections are presented and compared with available experimental and theoretical results. A prominent oscillatory energy dependence structure in the transfer-excitation cross sections is observed and explained by a strong competition between these channels and the projectile-excitation processes. Moreover, the angular-differential cross sections considered in this work exhibit an oscillatory structure which is interpreted within a Fraunhofer-type diffraction model. For the two highest considered collision energies, the cross sections show a different pattern for which both Fraunhofer-type diffraction and the Thomas mechanism have to be advocated.

Theoretical studies of dielectronic recombination and resonant transfer excitation for highly ionized Cu18+ ions

The dielectronic recombination cross sections of Cu18+ ions are calculated using a quasi-relativistic theory. The effects of configuration interaction to the cross sections are analysed. Base on the calculated data, the resonant transfer excitation cross sections of Cu18+ with H2 targets are estimated by using the convolution of Compton profile of the target electron in the impulse approximation. Good agreement of the present calculation with new experimental observation is achieved.

Measurement of depolarization of neon excited atoms due to helium atom collisions at low temperature

Disorientation and disalignment of neon excited atoms in the fine-structure 2p2 level (in Paschen notation) of the 2p53p configuration have been investigated in a helium–neon glow discharge at temperatures between 17 and 300 K. The determined rate coefficients for disorientation, Rdo, and disalignment, Rda, due to helium atom collisions show similar temperature dependences of T1.6 below 77 K with the ratio about Rdo:Rda = 2:1. This fact indicates that both the excitation transfer cross sections between the mJ = 1 and mJ = −1 states, σ1–1, and between the mJ = ±1 and mJ = 0 states, σ10, due to helium atom collisions have similar energy dependences of E1.1 with the ratio about σ1–1:σ10 = 2.5:1 below 10 meV.

Excitation transfer cross sections for levels of the Ne2p53dconfiguration

We have measured electron-impact optical emission cross sections for selected resonance and nearby nonresonant levels of the neon ${2p}^{5}3d$ configuration at pressures from 2 to 70 mTorr. The enhanced fluorescence observed from nonresonant levels at high pressures is consistent with excitation transfer from highly populated resonance levels of the ${2p}^{5}3d$ configuration. We measured excitation-transfer cross sections of $540\ifmmode\pm\else\textpm\fi{}80 {\mathrm{\AA{}}}^{2},$ $1600\ifmmode\pm\else\textpm\fi{}500 {\mathrm{\AA{}}}^{2}$, and $1100\ifmmode\pm\else\textpm\fi{}200 {\mathrm{\AA{}}}^{2}$ for transfer from the ${3d}_{5}\ensuremath{\rightarrow}{3d}_{6},$ ${3s}_{1}^{\ensuremath{'}}\ensuremath{\rightarrow}{3s}_{1}^{\ensuremath{''}},$ and ${3s}_{1}^{\ensuremath{'}}\ensuremath{\rightarrow}{3s}_{1}^{}$, respectively.

Electron-impact excitation and collisional transfer into thenFlevels of helium

Through the use of Fourier-transform spectroscopy, the electron-impact optical cross sections for the infrared emissions from the $4F,$ $5F,$ and $6F$ levels of the helium atom have been measured for gas pressures between 3 and 50 mTorr and incident electron energies from threshold to 200 eV. The very strong pressure dependence of these measured emission cross sections at different energies is in excellent quantitative agreement with the mechanism of excitation transfer from the ${n}^{1}P$ levels through collisions with ground-state helium atoms. From our data, we determine the electron-impact cross sections for excitation into the $4F,$ $5F,$ and $6F$ levels. The effects of the singlet-triplet mixing in the nF states on the excitation cross sections are discussed. The excitation data of the $5F$ level are compared with those of $5{}^{1}S$ and $5{}^{1}D.$ We have also obtained the $n{}^{1}\stackrel{\ensuremath{\rightarrow}}{P}\mathrm{nF}$ collisional excitation transfer cross sections for $n=4--7.$

Theoretical investigation of barium-helium collisions. II. The excitation transfer cross sections

We develop a theoretical description of collisions between an excited barium atom in the 6s6p{sup 3}P{sub J}, 6s5d{sup 1}D{sub 2}, or 6s5d{sup 3}D{sub J} state and a helium atom in the ground state. Using the adiabatic potential curves presented in the preceding paper, state-to-state cross sections are obtained from a fully quantum-mechanical close-coupling calculation using the generalized log-derivative approach. Our theoretical results are compared to recent experimental data. {copyright} {ital 1997} {ital The American Physical Society}

Depopulation cross sections for low lying states of barium

Cross sections for excitation transfer between the low lying states of barium are calculated using a semiclassical Landau-Zener model and compared with existing experimental and theoretical data.

The collision cross sections for excitation energy transfer in Rb*(5P 3/2) + K(4S 1/2) ? Rb(5S 1/2) + K*(4P J ) processes

The collisional excitation transfer for the processes Rb*(5P3/2)+K(4S1/2) → Rb(5S1/2)+K*(4P J ),J=1/2, 3/2, was investigated using two-photon laser excitation technique with a thermionic heat-pipe diode as a detector. The population densities of the K 4P J levels induced by collisions with excited Rb atoms as well as those produced by direct laser excitation of the potassium atoms were probed through the measurement of the thermionic signals generated due to the ionization of the potassium atoms emerging from the K(4P J ) → K(7S1/2) excitation channel. The measurements of the thermionic signals in addition to the spectroscopic determination of the potassium number density yield the following values for the excitation transfer cross sections: σ1(Rb 5P3/2 → K 4P1/2)=8 A2 and σ2(Rb 5P3/2 → K 4P3/2)=11 A2. The accuracy of the presented results is ∓15%. The obtained results are compared to those for the opposite processes K*(4P J )+Rb(5S1/2) → K(4S1/2)+Rb*(5P3/2).

Resonance excitation transfer between two dipole rotors for high rotational quantum numbers

A classical description of the resonance transfer of rotational energy between identical rigid rotors through the dipole—dipole interaction is formulated. It is shown that the sudden approximation provides a satisfactory approach for the calculation of the excitation transfer cross section in collisions of rotationally polarized molecules with high j quantum numbers. The resonance excitation transfer cross section for collisions of unpolarized molecules is calculated.

Fine-structure effects on resonant transfer excitation cross sections for Li-like-ion collisions with H2 and He.

We have calculated configuration-mixing LS-coupling and intermediate-coupling cross sections for the process of resonant transfer excitation followed by x-ray stabilization in collisions of the Li-like ions ${\mathrm{S}}^{13+}$, ${\mathrm{Ca}}^{17+}$, ${\mathrm{Ti}}^{19+}$, ${\mathrm{V}}^{20+}$, ${\mathrm{Ni}}^{25+}$, and ${\mathrm{Ge}}^{29+}$ with ${\mathrm{H}}_{2}$ and He. Fine-structure interactions preferentially enhance the KLL peak (by up to 35% for ${\mathrm{S}}^{13+}$) over the KLn (n>L) peak (+5%), and this largely resolves the discrepancy that exists between theoretical LS-coupling calculations and experiment over the relative height of the two peaks for the case of ${\mathrm{Ca}}^{17+}$+${\mathrm{H}}_{2}$.

Modifying excitation transfer cross sections with an ac Stark effect

We show that it is possible to manipulate electronic energy transfer collision cross sections between different atomic species by using a strong electromagnetic field close to resonance with a transition between two excited states to modify the energy levels (i.e., to create dressed states), which may be placed in or out of resonance with populated states (forming a population reservoir) in one of the species. We outline an estimate for a transfer cross section for a demonstration scheme and show that cross-section enhancements up to the order of 10(3) are possible.

The endothermic excitation transfer process Kr *( 3P j) + N 2(X) → Kr( 1S 0) + N 2(C): a sensitive probe for the 3P 2: 3P 0 population ratio

The excitation transfer cross section of the Kr *-N 2 system has been measured in a crossed-beam experiment in the energy range 0.5 ⩽ E(eV) ⩽ 2.7, using a mixed beam of metastable Kr * atoms. The strongly different threshold energies (0.47 and 1.12 eV for the values J = 0 and J = 2, respectively, of total electronic angular momentum of Kr *) allow for an analysis in terms of fine-structure-dependent cross sections Q J ( E), resulting in Q 0 (0.94 eV) = 3.38 Å 2 and Q 2(2.24 eV) = 2.46 Å 2 at twice the threshold energy. The slightly larger cross section for J = 0 can be explained qualitatively in terms of diabatic initial- and final-state potentials, coupled by two curve crossings with the ionic Kr +-N − 2 Coulomb potential. The large difference with the experiments of Tabayashi and Shobatake, resulting in Q 0 ≈ 10 −2 Q 2, is most likely due to a population ratio 3P 0: 3P 2 = 1:146 in their atmospheric-arc beam source which is far from the assumed ratio 1:5 of statistical weights.

Transfer excitation in He++He collisions studied by 0 degrees electron spectroscopy

Spectra of autoionisation electrons from helium projectiles have been measured in collisions of 50-500 keV 3He+ ions with He target atoms. Kinematic line broadenings were substantially reduced by observing electrons at 0 degrees with respect to the incident beam. Lines attributed to states such as 2s2 1S, 2s2p 3P,2p2 1D and 2s2p 1P were resolved. The 2s2p 3P state was found to be predominantly produced exhibiting an asymmetric lineshape (Fano profile) that arises from an interference between transfer excitation and electron capture to the continuum (ECC). Absolute transfer excitation cross sections were deduced from the data. The cross section for the production of the 2p2 1D state exhibits a maximum at about 350 keV projectile energy showing that electron-electron interaction may play an important role in the excitation process.

An experimental study of collisional transfer of electronic excitation, collisional dissociation of excited molecules and photodissociation in alkali vapour

Abstract A single-frequency laser is used to excite Na 2 molecules to the electronic B state. Besides the molecular fluorescence also atomic Na resonance radiation is observed. This is caused by: collisional transfer of electronic excitation from a Na 2 (B) molecule to a Na atom, collisional dissociation of Na 2 (B) molecules and photodissociation of Na 2 from very high vibration—rotation ( v. J ) levels of the ground state. We show how the contributions of these processes can be separated experimentally and characterized quantitativily over a wide range of temperatures, using a free-jet expansion. Illustrative results are given for one laser frequency (i.e. one molecular transition). Effective collision cross sections for excitation transfer and for collisional dissociation are given. The probability of photodissociation is compared to the probability of the discrete (B ← X) transition. The relaxation of the number of molecules in high ( v. J ) levels in a free jet is obtained.

Energy transfer of O( 1S) atoms in collision with O( 3P) atoms

Elastic scattering and excitation transfer collision cross-sections in O( 1S)-O( 3P) collisions are calculated. These cross-sections are needed in determining the degree of thermalization of the O( 1S) atoms in the nighttime thermosphere. A formula is given for the rate coefficient for the production of an O( 1S) atom with a specific energy in collisions involving an O( 1S) atom of a given initial energy and the ground state O( 3P) atoms of a thermal gas. Effective elastic scattering and excitation transfer cross-sections are defined and calculated to be 1.71 × 10 −15 cm 2 and 6.67 × 10 −16 cm 2 respectively at a relative collision energy of 0.41 eV.

Radiation diffusion and saturation in optically thick Na vapor

We have measured the time-dependent fluorescence of the sodium $D$ lines following pulsed excitation of one $D$ line, in the presence of radiation trapping with optical depths ${k}_{0}L$ of \ensuremath{\sim}10 to 2000. When collisional coupling of the $3{P}_{\frac{1}{2}}$ and $3{P}_{\frac{3}{2}}$ levels and different radiative-escape probabilities for each $D$ line are taken into account, we obtain excellent agreement with Holstein's theory for the effective radiative decay rates in the Doppler region (${k}_{0}L\ensuremath{\sim}10\ensuremath{-}300$) and in the redistributed Lorentzian region (${k}_{0}Lg1000$). For ${k}_{0}L$ between these two regions, we observe an abrupt transition between the two limiting formulas. The buildup rate of the sensitized fluorescence signal also yields a $3{P}_{\frac{3}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ excitation transfer cross section in agreement with our previously reported cw measurement. Additionally, we have measured the dependence of the $3P$ fluorescence on laser power and beam diameter, and we explain the observed approach to saturation. We suggest that the previously reported anomalous approaches to saturation may be explained in terms of the laser beam burning through the optically thick vapor. Laser-beam spatial intensity variations and self-focusing also contribute to fluorescence signals that deviate from the usual single-atom saturation behavior.

A crossed-beam time-of-flight study of metastable helium in collisions with helium, neon and argon

Absolute velocity-resolved total integral cross sections have been measured for collisions of helium singlet (21S0) and triplet (23S12) metastable (He*) atoms with ground-state helium, neon, and argon atoms in the thermal velocity range of 1.0-3.5*105 cm s-1. Additional measurements on the He*-Ne system with a large input aperture at the detector to suppress the sensitivity of the apparatus to elastic scattering failed to show the previously predicted (Arathoon) sharply rising velocity structure in the inelastic excitation transfer cross sections.

Penning Ionization of Ar and N2by He(23S) and Excitation Transfer from Ar(1.3P) to N2

Chemi-ionization cross sections of Ar and N 2 by He(2 3 S) and excitation transfer cross sections from the first excited configuration of Ar to N 2 have been measured by monitoring emissions from N 2 at 600 K. Helium and argon atoms are excited by an electron beam of energy close to the energy of the level concerned and any contributions from higher excited states can be excluded. Cross sections obtained are 9.4 +1.3 -1.7 , 7.9 -0.7 +1.2 , 29 -3 +5 and 8.7 -0.8 +1.3 Å 3 for He(2 3 S)-Ar, He(2 3 S)-N 2 , Ar( 1 P 1 )-N 2 and Ar( 3 P)-N 2 , respectively. The values are larger than other measurements at room temperature and are consistent with an energy dependent cross section.

Charge transfer excitation in low energy collisions between rare gas ions and cadmium atoms

Excited state production in collisions between rare gas ion and cadmium atoms has been studied in a crossed beam apparatus at kinetic energies as low as 2 eV. It was found that charge transfer excitation cross sections resulting from impact of metastable argon ions were considerably larger than those with other rare gas ion reactants, either ground or excited state. The selective population of CdII (6s 2S) in these low energy Ar+/Cd interactions suggests that, under the proper conditions, laser action at 257.3 and/or 274.9 nm might be possible.

82D3/2 ↔ 82D5/2 excitation transfer in rubidium induced in collisions with ground-state Rb atoms

Cross sections for inelastic transfer between the 82D3/2 and 82D5/2 fine-structure states in rubidium, induced in resonant collisions with ground-state Rb atoms, have been determined using an experimental method involving two-photon excitation of atomic fluorescence. Rubidium vapor in a fluorescence cell was irradiated with pulses of 641 nm radiation from a N2 laser-pumped dye-laser tuned to excite one of the 82D states. The resulting fluorescence included the direct component originating from the optically excited state and a sensitized component arising from the other fine-structure state populated by collisions. Relative intensities of the fluorescent components, determined by photon-counting techniques, yielded the cross sections for excitation transfer: Q(2D3/2 → 2D5/2) = 8.1 × 10−13 cm2; Q(2D3/2 ← 2D5/2) = 5.5 × 1013 cm2; as well as [Formula: see text], the effective quenching cross section. The excitation transfer cross sections which are considered accurate to within ±20% are in the ratio predicted by the principle of detailed balancing.